1. Field of the Invention
The present invention relates to a solid-state imaging device capable of focus detection of a phase detection method and imaging a parallax image for stereoscopic viewing and a digital camera comprising the solid-state imaging device.
2. Description Related to the Prior Art
Digital cameras comprising a taking lens and a CCD-type or CMOS-type solid-state imaging device are widely used. It is common that such digital cameras comprise an autofocus (hereinafter referred to as the AF) function to automatically adjust the focus of the taking lens.
A so-called phase-detection AF function is known (see Japanese Patent Laid-Open Publication No. 2006-105771, U.S. Pat. No. 6,829,008 (corresponding to Japanese Patent Laid-Open Publication No. 2000-156823), and Japanese Patent Laid-Open Publication No. 2010-093619). The phase-detection AF function automatically adjusts the focus of the taking lens based on the focus detection of a phase detection method. The phase-detection AF function is achieved by providing first phase difference pixels (hereinafter simply referred to as the first pixels) and second phase difference pixels (hereinafter simply referred to as the second pixels), in addition to normal pixels for imaging a normal image (two-dimensional still image).
The normal pixel has a normal opening section that is coincident with the center position of the light-receiving surface of a photodiode. The normal pixel receives light incident on the photodiode through the normal opening section. The first pixel has a first off-center opening section that is shifted in a first direction relative to the center position of the light-receiving surface of the photodiode. The first pixel receives light incident on the photodiode through the first off-center opening section. The second pixel has a second off-center opening section that is shifted in a second direction relative to the center position of the light-receiving surface of the photodiode. The second direction is opposite to the first direction. The second pixel receives light incident on the photodiode through the second off-center opening section.
The normal opening section, the first off-center opening section, and the second off-center opening section are formed through a light shield film. The light shield film covers over a semiconductor substrate in which the photodiodes are formed. The sizes of the first and the second off-center opening sections are smaller than that of the normal opening section.
The first pixel is highly sensitive to light incident from a direction of the shift of the first off-center opening section from the center position of the light-receiving surface of the photodiode. The same holds true for the second pixel. For example, the first pixel is highly sensitive to the incident light from upper right. The second pixel is highly sensitive to the incident light from upper left.
The image obtained from the first pixels and the image obtained from the second pixels shift from each other in a right-left direction in accordance with a focal state of the taking lens. An amount of shift between the two images corresponds to an amount of shift of the focus of the taking lens. The two images coincide and the amount of shift is zero when the taking lens is in focus. The amount of shift between the two images increases as the amount of shift in focus of the taking lens increases. Hence, a focus adjustment amount of the taking lens is determined by detecting the directions of the shifts of the respective images obtained from the first and second pixels and the amount of shift between the images.
During framing of a subject, the digital camera comprising the phase-detection AF function performs the AF control based on pixel signals from the first and the second pixels. In capturing a normal image, the digital camera uses all of the normal pixels, the first pixels, and the second pixels to produce image data.
In the solid-state imaging device disclosed in the above-described documents, a part of the photodiode of the first pixel is covered by the light shield film because the first off-center opening section of the first pixel is shifted from the center position of the light-receiving surface of the photodiode. Hence, the light traveling toward the area covered with the light shield film is blocked by the light shield film. The same holds true for the second pixel. For this reason, the amount of light incident on each of the photodiodes of the first and the second pixels is less than that incident on the photodiode of the normal pixel, and the sensitivity of each of the first and second pixels is lower than that of the normal pixel.
In the solid-state imaging device disclosed in the above-described documents, the direction of the light incident on the first pixel is opposite to the direction of the light incident on the second pixel. Hence, in capturing a normal image, it is necessary to perform so-called pixel addition in which pixel signals of the adjacent first and second pixels are added. In this case, a pixel signal is composed of signals from a pair of first and second pixels, so that resolution of the normal image decreases.